Estimation of individualized counting efficiency by means of numerical simulation on the assessment of iodine-131 activity in thyroid

摘要

It is necessary to calibrate the thyroid monitoring system to properly assess internal contamination in thyroid. The conventional calibration performed by the neck physical phantom is widely used for calibrating monitoring systems, but the neck physical phantom cannot reflect the individual characteristic such as the size of thyroid, the position of thyroid, and overlay tissue thickness. The physical variations between the neck physical phantom and the human body anatomy can influence on the measurement of radioiodine in thyroid, especially the counting efficiency. Therefore, this study describes the estimation of counting efficiency considering individual characteristics of subject in thyroid measurement for iodine-131 by using the Monte Carlo technique. In this study, the NaI(Tl)-based and high purity germanium-based thyroid monitoring systems in operation at the Korea Institute of Radiological and Medical Sciences were used and embodied with Monte Carlo code. Based on the computational modelling of monitoring systems, simulations were performed using the ICRP adult reference computational phantoms and the UF/NCI pediatric computational phantoms (1, 5, 10, and 15 years) to consider the anatomical variations, including the age and gender of subjects in thyroid measurement. The overlay tissue thickness of subject could be estimated from the counting ratio of two energy peaks at 80.2 and 364 keV of iodine-131 due to the attenuation of gammas. Thus, the overlay tissue thicknesses of each phantom were modified to evaluate the relationships between the above counting ratio and counting efficiency. From these relationships, the equations for estimating the individualized counting efficiencies for each age and gender group were derived. Through these equations, it is possible to estimate the individualized counting efficiencies with considering the age, gender, and overlay tissue thickness of subject in thyroid measurement for iodine-131.